The present invention relates to radioactive logging well logging tools and more particularly to carrier apparatus for transporting and deploying a radioactive sensor package into a well borehole for logging purposes.
In obtaining certain radioactive logging data from the formation surrounding an oil or gas well borehole, it is necessary to maintain the radioactive source in contact with the borehole wall to prevent scattering of the radioactive energy into the borehole annulus instead of the formation. This is especially true in making gamma ray radiation measurements of the formation to determine bulk density and photoelectric absorption index in which a sensor package is used that includes a gamma ray source and a pair of spaced scintillation detectors to measure the scattering and absorption effects of the formation on the gamma ray energy entitled by the source. Quality measurements for this type of logging instrument requires that the sensor package maintain constant contact with the borehole wall during the logging trip up the borehole. If the borehole wall has an extremely rugose condition, the sensor pad or package may tend to lift off from the wall and lose direct contact with the borehole wall. If the sensor pad or package loses contact with the formation comprising the borehole wall, some of the gamma ray energy will be diverted into the borehole annulus instead of the formation, resulting in less accurate measurements with a high degree of uncertainty. In addition, many existing radioactive logging tools cannot traverse small diameter boreholes (to a minimum of 4.5 inches) because of the tool carrier diameter dictated by the operation of and the sensor pad package and the arm extension means for extending the sensor package into contact with the borehole wall.
Yet another feature of the present invention is to provide carrier apparatus for a radioactive logging sensor pad that includes decentralizing arms extendable into contact with the borehole wall for decentering the axis of the tool in the direction of extension of the sensor pad assembly having force loading applied to the decentralizing arms for maintaining constant contact between the borehole wall and the decentralizing arms to accommodate known borehole diameters.
Accordingly, one feature of the present invention is to provide carrier apparatus for a radioactive sensor package having linkage means that may be actuated to extend the sensor pad laterally into contact with the borehole wall, the linkage means providing preselected force loading for linkages interconnecting the ends of the sensor pad to the tool body for providing articulated movement of the sensor pad with respect to the axis of the tool body in order for the sensor pad to maintain constant contact with the borehole wall under rugose conditions.
Another feature of the present invention is to provide a rugosity caliper for measuring the rugosity of the borehole wall along the longitudinal path to be traversed by the sensor pad or package.
Still another feature of the present invention is to provide a tool body or carrier for a radioactive sensor pad assembly that when in its actuated condition can traverse boreholes with diameters below 5 inches.